In oil production it is necessary to monitor boreholes with regard to pressure and temperature. In the borehole, the liquid pressures can be up to approximately 100 MPa (1000 bar) and the temperatures can be up to above 200° C. Electrical sensors, such as e.g. piezoresistors, piezoelectric elements, capacitive probes or crystal resonators or optical pressure sensors, such as e.g. Fabry-Perot resonators, or elasto-optical sensors, are often used for pressure measurement up to approximately 170° C.
U.S. Pat. No. 5,844,927 discloses a DFB fiber laser sensor in accordance with the preamble of the independent claims. What is disclosed is a laser-amplifying fiber with a distributed fiber Bragg grating resonator in which two orthogonally polarized modes can oscillate with slightly different wavelengths. In the case where the laser-amplifying fiber is used as a sensor fiber, as a result of a transverse force, a birefringence is induced between the polarization modes and is measured as force-proportional beat frequency. Spectrally separate serial and parallel multiplex arrangements of a plurality of fiber laser sensors are also specified. What is disadvantageous is that use is made exclusively of two polarization modes, namely the spatial fundamental mode of the fiber with its two orthogonal linear polarization states, and an isotropic pressure cannot be measured.
WO 99/44023 discloses a fiber laser pressure sensor in which a laser-amplifying fiber and a sensor fiber are arranged between two fiber Bragg grating end reflectors. The sensor fiber has an asymmetry in such a way that, as a result of isotropic pressure, a birefringence can be induced between two linear polarization modes or between the LP01, and LP11even spatial modes and can be measured as pressure-proportional beat frequency. In accordance with WO 00/39552 in such an arrangement, differential pressures can also be measured by means of two sensor fiber segments rotated by 90° with respect to one another. Disadvantages of these sensors include the limited dynamic range, since induced optical phase shifts can be unambiguously detected only up to a maximum value of 90°, and the low stability of the laser emission. In addition, the fabrication is complicated because the fiber segments have to be coordinated with one another and be arranged in separate pressure chambers.